The present invention relates to field devices operating in the process control and measurement industry. In particular, the present invention relates to field devices with improved diagnostic capabilities.
Field devices, such as process variable transmitters, are used by a number of industries to remotely sense a process variable. A controller may then transmit control information to another field device, such as a valve, in the process to modify a control parameter. For example, information related to pressure of a process fluid may be transmitted to a control room and used to control a valve in an oil refining process. Other examples of a field devices include handheld configuration and/or calibration devices.
One of the relatively recent advances in process control and measurement has been the implementation of all-digital process communication protocols. One example of such an all-digital communication protocol is FOUNDATION Fieldbus. Fieldbus is directed to defining a communication layer or protocol for transmitting information on a process communication loop. The Fieldbus protocol specification is ISA-S50.02-1992, promulgated by the Instrument Society of America in 1992. Fieldbus is a process industry communications protocol described in the Fieldbus technical overview Understanding FOUNDATION™ Fieldbus Technology (1998) available from Rosemount Inc. in Eden Prairie, Minn. As used herein, “fieldbus” is intended to mean any communication protocol operating in accordance with the ISA-S50.02-1992 specification and equivalents thereof, process communication protocols that are backwardly compatible to the ISA-S50.02-1992 protocol, and other standards operating in accordance with International Electrontechnical Commission (IEC) Fieldbus Standard 61158. For example, for the purposes of this patent document, Profibus, ControlNet, P-Net, SwiftNet, WorldFIP and Interbus-S, is considered a fieldbus.
Advantages of fieldbus include relatively high-speed digital communication as well as signaling levels that facilitate compliance with intrinsic safety as set forth in APPROVAL STANDARD INTRINSICALLY SAFE APPARATUS AND ASSOCIATED APPARATUS FOR USE IN CLASS I, II AND III, Division 1, Hazardous (Classified) locations, Class No. 3610, promulgated by Factory Mutual Research, October, 1988. Industrial processing environments that require intrinsic safety compliance provide an added challenge for electrical instrumentation and automation of the process control system since such environments may contain flammable or explosive vapors. Accordingly, process communication loops operating in such processing environments are typically energy-limited. Multiple redundant circuits are used to ensure that energy levels on the communication loop are below a safe energy level so that the energy cannot ignite the flammable vapors, even under fault conditions. Field devices in such environments are also generally energy limited as well. Process communication loops that pass through the safe area of the flammable processing environment to outside equipment such as a controller typically pass through energy limiting barriers such as an intrinsic safety barrier so that a fault occurring outside the flammable environment will not generate a spark inside the frequently explosive fluid processing environment. Process communication loops that have the potential for higher level signals that could spark under fault conditions are often not permitted to pass through or connect to equipment in a flammable processing environment.
In some digital process measurement installations, all field devices communicate over essentially the same digital process communication loop. In such cases, it is much more important to diagnose problems before they become critical and affect the operation of the loop. For example, should a single device fail and begin to draw too much energy, the signaling levels on the process communication loop could collapse thereby inhibiting all communication over the loop and effectively causing the system to fail.
While fieldbus has proved to be an advance in the art of process control and measurement, the nature of its all-digital communication in applications which are relatively intolerant of faults, drives an ongoing need for enhanced diagnostics, not only for the fieldbus devices themselves, but for the process control system in general.